1. Field of the Invention
The present invention relates to an antimicrobial agent against acid-resistant and heat-resistant bacteria, and more specifically, to an antimicrobial agent including hop acids which inhibits the growth of acid-resistant and heat-resistant bacteria such as Alicyclobacillus acidoterrestris and Alicyclobacillus acidocaldarius. 
2. Description of the Related Art
Alicyclobacillus acidoterrestris and Alicyclobacillus acidocaldarius are acid-resistant and heat-resistant bacteria that cause the spoilage of fruit juice. One of the characteristics of such spoilage is the production of phenolic compounds that affect the taste of the fruit juice as well as clouding of the fruit juice. Because Alicyclobacillus acidoterrestris and Alicyclobacillus acidocaldarius are acid-resistant and heat-resistant, conventional techniques for bacteria control such as pasteurization and pH reduction are not completely effective in destroying these bacteria. For example, the spores of these bacteria possess resistance against the normally employed pasteurization method for fruit juice. As a result, hot sterile fill and severe pasteurization methods have been used in an effort to destroy these bacteria. Unfortunately, these methods may alter the true to type character of the fruit juice due to the severe heat.
In order to suppress the growth of Alicyclobacillus acidoterrestris and Alicyclobacillus acidocaldarius in fruit juice, it is reported in U.S. Pat. No. 6,329,011 that nisin, which is a peptide derived from lactic acid bacterium, may be added to fruit juice. This patent also proposes using thionins, which are peptides derived from wheat and barley, to suppress the growth of Alicyclobacillus acidoterrestris and Alicyclobacillus acidocaldarius in fruit juice.
Even though these known methods for suppressing the growth of Alicyclobacillus acidoterrestris and Alicyclobacillus acidocaldarius in fruit juice are available, there is still a need for alternative methods for suppressing the growth of Alicyclobacillus acidoterrestris and Alicyclobacillus acidocaldarius in fruit juice. It would be desirable if such methods would avoid severe heat treatment of the fruit juice so that the natural flavor of the fruit juice is not degraded.